In microorganic butanol fermentation, solvents such as acetone and ethanol and organic acids such as butyric acid, acetic acid and lactic acid are produced as by-products in addition to butanol. These by-products are a factor of lowering butanol yield and an obstacle in constructing a process for collecting butanol.
In butanol fermentation, decreasing by-products (e.g., acetone, ethanol, organic acids) is very useful for improving the yield of butanol in relation to glucose and simplifying a separation/purification step. In the circumstances, studies for decreasing by-products have been made. As a method therefor, disruption of by-product synthetic genes or enhancement of butanol synthetic genes have been frequently adopted and carried out by means using homologous recombination and mutation. For example, in Non Patent Literature 1, a buk (butyrate kinase) gene and a pta (phosphotransacetylase) gene are disrupted by homologous recombination. However, in this method, acetic acid production increases significantly when the butyric acid production pathway is disrupted and butyric acid production increases significantly when the acetic acid production pathway is disrupted. In addition, there is a case where production of a solvent increases as in the case where aad (alcohol/aldehyde dehydrogenase) gene is cloned and thereafter excessively expressed (Non Patent Literature 2). There is another case where gene suppression and introduction are simultaneously performed. In Non Patent Literature 3, thl (acetyl-CoA dimerized enzyme) gene and aad gene are expressed while expression of ctfB (subunit B of CoA transferase) gene is suppressed thereby the concentration of ethanol is increased to about 13 g/L.
As a case in which ethanol and butanol are produced without producing acetone, Non Patent Literature 4 discloses simultaneous disruption of adc (acetoacetate decarboxylase) gene or ctfAB (CoA transferase) gene and pta gene. In that case, butyric acid accumulates significantly whereas production of butanol decreases drastically. Patent Literature 1 describes that a strain in which buk or ptb (phosphotransbutyrylase) gene of Clostridium acetobutyricum is disrupted such that butyric acid production pathway is disrupted is further subjected to adjustment of acetone, lactic acid, acetic acid and hydrogenase production. However, specific numerical values thereof are not disclosed in the literature. In Non Patent Literature 5, ptb and pta genes and buk and pta genes are disrupted in Clostridium acetobutyricum; however, a decreasing effect in production of by-products including butyric acid and acetic acid is not sufficient and an increase in butanol yield has not been achieved.
Accordingly, it has been desired to develop a microorganism capable of producing butanol with high efficiency while suppressing production of by-products such as butyric acid, acetic acid and ethanol.